Abstract: Martensite is one of the most important structures determining the ability to tailor the performance of steel and several other engineering materials. In previous studies, the face-centered cubic (FCC) to hexagonal close-packed (HCP) (or body-centered cubic (BCC)) martensitic transformation and the FCC to body-centered tetragonal (BCT) (or BCC) martensitic transformation during deformation and quenching were widely investigated; these transformations usually improve the plasticity and strength of steel, respectively. In this work, detailed transmission electron microscopy (TEM) observations and electron diffraction pattern analyses of the BCT and BCC twins are performed along the [110], [131], and [$\overline{1}$53] zone axes. The transformation of BCT twins into BCC twins along the [131] zone axis is also analyzed through in situ heating during the TEM observations. The high-resolution TEM observations of mutually perpendicular BCT variants are consistent with the atomic arrangement obtained for the quenching pathway during the martensitic transformation. In addition, different orientation relationships between austenite and martensite are explained in terms of the atomic migration occurring during the deformation pathway and the quenching pathway of the martensitic transformation.

Key words: Body-centered tetragonal twin, Martensitic transformation, Steel, Transmission electron microscopy, Orientation relationship